1. Field of the Invention
The disclosure relates to a method and a device for increasing the dynamic range and measuring accuracy of a measuring device for spectrum and/or network analysis of high frequency signals.
2. Related Technology
In measuring devices for spectrum analysis and/or network analysis of high-frequency electronic signals, for example, spectrum analyzers or network analyzers, which operate according to the principle of superimposition, the high-frequency measuring and generator signals are frequency-translated through mixers from the high-frequency range into the intermediate-frequency range. In this context, the entire relevant high-frequency range is imaged into the intermediate-frequency range for further analysis by tuning the mixer with a variable-frequency mixer signal of a local oscillator.
Alongside its frequency bandwidth and frequency resolution, the performance capability of a measuring device of this kind is characterized primarily by the dynamic range it can realise. The dynamic range results from the capability of the measuring device to measure and present at the same time signals with very low power in the presence of signals with very high power. The dynamic range of a measuring device of this kind is narrowed by interference signals, which are undesirably superimposed within the measuring device either over the measuring signal to be analyzed or over the generator signal to be generated.
These system-specific interference signals are attributable to different interference sources. A considerable interference factor is provided by noise—phase noise from the local oscillator, noise from the receiver stage and the mixer, which, in fact, provides a comparatively low interference level, but is more difficult to compensate because of its stochastic character. A further important interference source is presented by nonlinear functional units in the measuring device—mixers and analog-to-digital converters, which lead to nonlinear distortions of the measuring and generator signal recognizable in additional spectral lines within the frequency range to be analyzed. Finally, the useful signals—measuring signals to be analyzed and generator signals to be generated—are contaminated by high-frequency signals generated in the measuring device—reference frequencies, clock frequencies and network frequencies, which are coupled into the signal paths of the measuring signal or generator signal to be mixed down into the intermediate-frequency range or the signal paths of the mixer signal. Moreover, this high-frequency interference is not adequately attenuated by the filters integrated in the individual signal paths, because these paths are generally very broad-band in design for various reasons, such as the high dynamic requirement of the phase-control loops and the high threshold frequency of the mixer signal.
In principle, a distinction can be made between interference signals, of which, at the frequencies Fzi according to FIG. 1, each spectral line provides a fixed frequency spacing Δf relative to the frequency FLO of the mixer signal; and interference signals, of which the spectrum according to FIG. 2 is absolutely fixed and independent of the frequency FLO of the mixer signal. The first group of interference signals includes, for example, coupled high-frequency signals, which modulate the mixer signal in the mixer, or the spectral components of the higher-order intermodulation products generated by nonlinear distortion. The second group of interference signals includes, for example, high-frequency signals of the frequency divider integrated in the individual signal paths of the measuring and generator signals coupled at the output of the mixer.
In order to improve the dynamic range of a measuring device of this kind, these system-specific interference signals must be separated within the frequency range to be analyzed from the useful signals—measuring signals to be analyzed and generator signals to be generated and/or completely removed from the frequency range to be analyzed.
In the spectrum analyzer described in EP 0 841 569 B1, a measuring signal to be analyzed is mixed in several measurements by means of a mixer into several frequency ranges, which are frequency-translated relative to one another, converted by an analog/digital converter and then supplied to a Fourier transformation. Alongside the useful-signal spectral components frequency-translated by the relevant mixer frequency, the individual spectra obtained from the Fourier transformation also contain the interference-signal spectral components generated, for example, in the mixer or in the analog-to-digital converter, which are not subjected to any frequency translation. In a frequency-translation unit, all of the individual spectra generated in the Fourier transformation are then translated back by the frequency value, with which they were frequency-translated in the mixer. If the spectra frequency-translated in this manner are averaged with reference to frequency unit in an averaging unit, then the useful-signal spectral components will be retained because of their identical frequency position in the individual spectra, while the interference-signal spectral components are attenuated because of their statistical distribution in the individual spectra.
The disadvantage with this spectrum analyzer is the fact that the interference-signal spectral components are only attenuated by the averaging unit, but are not removed from the frequency range to be analyzed.
The switching arrangement of U.S. Pat. No. 4,791,577 also provides a frequency translation of the measuring signal to be analyzed into several frequency ranges—in this case, two frequency ranges—in a mixer, a subsequent analog-to-digital conversion of each frequency-translated measuring signal and a generation of the spectrum belonging to the relevant frequency-translated measuring signal through a Fourier transformation. The two spectra of the differently frequency-translated measuring signal are transferred back via a shift register into an identical frequency position, in which the useful-signal spectral components of the two spectra come to be disposed at identical frequencies, while the interference-signal spectral components of the two spectra are disposed at different frequencies. The switching arrangement of U.S. Pat. No. 4,791,577, differs from the arrangement of EP 0 841 569 B1, in that, by comparing the two spectra, the comparator generates a new spectrum, which now contains only useful-signal spectral components but no interference-signal spectral components.
The switching arrangement of U.S. Pat. No. 4,791,577 provides the disadvantage that the interference-signal spectral components in the spectrum of the measuring signal to be analyzed are blanked out only with the frequency resolution of the Fourier transformation. In this arrangement, interference-signal spectral components, which have a frequency spacing relative to the useful-signal spectral components, which is smaller than this frequency resolution, are superimposed over the useful-signal spectral components and, in the comparator, lead to an erroneous blanking out of the relevant useful-signal and interference-signal spectral component.